Electrochromic devices are devices in which a physical/chemical change produced in response to the induced electric field results in a change in the reflective (or transmissive properties) of the device with respect to electromagnetic radiations, e.g., uv, visible and IR radiations. Such devices, one embodiment being shown as item 10 in FIG. 1, generally comprise a film of electrochromic material 12 and an ion-conductive insulating layer 14 which functions as an electrolyte layer. The film and the electrolyte layer are in surface contact with each other for exchange of ions between the electrochromic film and the electrolyte layer. Two conductive electrode layers, 16 and 18 in FIG. 1, at least one of them being transparent, are disposed on the opposite outer surfaces of the electrochromic material film and the electrolyte layer to provide means for applying a voltage across the combined thickness of the electrochromic film and the electrolyte layer. The electrode layers, 16 and 18 in FIG. 1, are provided on substrates, 20 and 22 of FIG. 1, which substrates may be of a material such as glass. Depending on the ion providing and ion storage capacity of ion conductive layer 16, a counter electrode located between ion conductive layer 14 and electrode layer 18 may be used. The electrodes are provided with external electrical leads 24 and 26 connected to a voltage providing source 28. Application of a voltage of proper polarity across the electrodes causes coloration of the electrochromic layer. By reversing the polarity of the applied voltage, the colored electrochromic layer will be uncolored (bleached). Changing from the bleached state to the colorless state or from the colored state to the bleached is termed "switching". The electrochromic material may be persistent in either its colored state or its non-colored state. By "persistent" is meant the ability of the material to remain, after removal of the electric field, in the absorptive state to which it is changed, as distinguished from a substantially instantaneous reversion to the initial state. The length of time a material is persistent is called its "open circuit memory" or simply "memory". Electrochromic devices of this type have been described for several uses, such as image display, for light filtering, etc. See, e.g., U.S. Pat. Nos. 3,708,220, 4,194,812; 4,278,329; 4,645,308; 4,436,769; 4,500,878; 4,150,879; 4,652,090; 4,505,021; and 4,664,934.
In such devices, the electrochromic film usually comprises an inorganic metal oxide material, most commonly a transition metal oxide, in particular: tungsten oxide. When tungsten oxide is the electrochromic material, the electrolyte layer is adapted to provide a positively charged light metal cation, preferably, a proton or a lithium ion.
The electrolyte layer is generally a liquid electrolyte solution, typically based on sulfuric acid or lithium perchlorate in propylene carbonate. However, use of a liquid electrolyte has the inherent disadvantage associated with containment of a fluid. That is, it is required with liquid electrolytes used in layered electrochromic devices that the edges of the device be sealed so as to retain the liquid electrolyte. U.S. Pat. No. 3,708,220, proposes to overcome such shortcomings by the use of gelled, sulfuric acid-polymeric electrolytes such as H.sub.2 SO.sub.4 -PVA (polyvinyl alcohol). It is taught therein, that such a gel electrolyte possesses good stability, high viscosity and transparency. Although liquid and gel electrolytes, such as those described in the foregoing patent, may impart good electrochromic performance, problems related to handling and containment of the liquid or gel remain. In addition, the preferred tungsten oxide electrochromic material as well as certain electrode materials are attacked by acidic electrolyte materials, limiting the utility of strong acids for this application.
Another proposed class of electrolytes, i.e., in addition to liquid and gel electrolytes, is solid electrolytes. U.S. Pat. No. 4,256,379 discloses a solid electrolyte of complex halides, particularly iodides, of silver with alkali metal or quaternary ammonium ions, e g., RbAg.sub.4 I.sub.5. According to the patent teachings, this electrolyte itself is used in contact with an electrode capable of providing ions which are the same as the "fast" ions of the conductor. The "fast" ion is preferably an alkali metal, copper or silver ion, silver being preferred. Additionally, this patent teaches solid electrolytes comprising aluminum compounds such as sodium beta-alumina and potassium beta-alumina. However, these electrolytes are all typically expensive to prepare and, in the case of the alumina compounds, could not be formed directly on components of an electrochromic device since they require very high processing temperatures. U.S. Pat. No. 4,491,392 proposes forming a solid electrolyte comprising a sheet of porous glass impregnated with a solid, ion-conductive silver or alkali metal compound. One disadvantage of employing such an impregnated glass sheet is that, because it is a solid of limited flexibility, it would be difficult to assemble the component layers of an electrochromic device and achieve the intimate contact required between this sheet and the adjacent layers. In "A New Family Of Organically Modified Silicates Prepared from Gels" by D. Ravine et al, Journal of Non-Crystalline Solids 82(1986) 210-219, techniques are disclosed for making lithium conducting solid electrolytes by a sol-gel process. The sol-gel can be made from a mixture of tetramethoxysilane and polyethlene glycol in non-aqueous solvents, e.g., methanol. One disadvantage of such a system is that it has limited flexibility and solidification can take days.
C. K. Chiang et al, In "Synthesis Of Ionic Conducting Interpenetrating Polymer Networks", Polymer Communications, 1987, Vol. 28, Feb., disclose an ionically conducting opaque white solid which comprises a continuous phase of a liquid poly(ethylene oxide)-salt complex in a continuous phase of an amine-crosslinked epoxy phase. The polyethylene oxide is the ion conducting medium, however, the ionic conductivity in polyethylene oxide drop dramatically below its freezing point. Thus, one disadvantage of this polyethylene oxide-salt/epoxy system is that the ionic conductivity of this system at low temperatures is limited by the freezing point of the liquid polyethylene oxide.
It would be highly desirable to provide an electrolyte useful in an electrochromic device, which electrolyte would have substantial flexibility so as to aid in the fabrication of electrochromic devices. At the same time, it would be highly desirable that the electrolyte be a solid so as to avoid problems associated with a liquid or gel electrolyte, e.g., containment and loss of ionic conductivity at low temperatures. It would also be desirable to provide a flexible, solid electrolyte having excellent ionic conductivity for alkali metal ions.
The aforementioned problems of prior art electrolytes are overcome by the flexible, solid electrolyte of the present invention.